https://bugzilla.mozilla.org/show_bug.cgi?id=1598826
If getting errors in output of gtk program like;
(__main__.py:16414): Gtk-WARNING **: 14:38:54.184: Theme parsing error: colors.css:71:44: Invalid number for color value
Open ~/.config/gtk-3.0/colors.css
At the very end of the file there are a number of lines that include rgb() fill in with 3 values;
@define-color theme_titlebar_background rgb(71,80,87);